This invention relates to the recovery of nickel from nickel bearing sulphide minerals.
Commercial bioleach plants which are currently in operation treating sulphide minerals, typically operate within the temperature range of 40xc2x0 C. to 50xc2x0 C. and rely on sparging air to the bioleach reactors to provide the required oxygen. Operation at this relatively low temperature and the use of air to supply oxygen, limit the rate of sulphide mineral oxidation that can be achieved.
The use of high temperatures between 50xc2x0 C. and 100xc2x0 C. greatly increases the rate of sulphide mineral leaching.
The solubility of oxygen is however limited at high temperatures and the rate of sulphide mineral leaching becomes limited. In the case of using air for the supply of oxygen, the effect of limited oxygen solubility is such that the rate of sulphide mineral leaching becomes dependent on and is limited by the rate of oxygen transfer from the gas to the liquid phase.
The bioleaching of nickel sulphide bearing minerals is similarly problematic and to the applicant""s knowledge no commercial nickel bioleaching plants are in operation.
According to one aspect of the invention there is provided a method of recovering nickel from a nickel bearing sulphide mineral slurry which includes the steps of:
(a) subjecting the slurry to a bioleaching process,
(b) supplying a feed gas which contains in excess of 21% oxygen by volume, to the slurry, and
(c) recovering nickel from a bioleach residue of the bioleaching process.
If the slurry contains copper then preferably copper is removed from the bioleach residue before recovering nickel therefrom.
The method may include the step of recovering cobalt from the bioleach residue before recovering nickel therefrom.
Iron may also be removed from the bioleach residue before recovering nickel therefrom. The iron may be precipitated from the bioleach residue by the addition of limestone to the residue, or in any other suitable way. Carbon dioxide generated in the iron precipitation step may be fed to the feed gas of step (b) or directly to the slurry.
In step (c) nickel may be recovered using any appropriate technique. Use may for example be made of a solvent extraction and electrowinning process. In this instance oxygen generated during the nickel electrowinning step may be fed to the feed gas of step (b) or directly to the slurry.
It is also possible to recover nickel, in step (c), using a pressure hydrogen reduction process. In this case nitrogen, produced during the generation of the feed gas which is supplied to the slurry in step (b), may be used for purging autoclaves used for nickel powder production in the pressure hydrogen reduction process.
The introduction of pressure acid leaching (PAL) for the recovery of nickel from lateritic ores opens up additional opportunities for bioleaching of nickel bearing sulphides. The volumes arising down stream of PAL processes are roughly an order of magnitude higher than the mass of nickel sulphide concentrates arising from sulphide nickel mines (approximately 1-3 million tonnes per annum versus approximately 20-200 thousand tonnes per annum).
Since residues from both processes are similar in nature (low pH, solubilised nickel and iron) it is advantageous to treat the bioleach residue for nickel and cobalt recovery using the larger PAL downstream process equipment Thus feeding a nickel sulphide bioleach residue slurry into a PAL residue slurry and treating both streams together thereafter would bring about considerable economies of scale in terms of capital and operating costs.
Thus the invention also extends to operating the aforementioned method in parallel to a process for recovering nickel from lateritic ores by pressure acid leaching to produce a nickel laterite residue slurry and then adding the bioleach residue to the nickel laterite ore slurry before carrying out step (c).
As used herein the expression xe2x80x9coxygen enriched gasxe2x80x9d is intended to include a gas, e.g. air, which contains in excess of 21% oxygen by volume. This is an oxygen content greater than the oxygen content of air. The expression xe2x80x9cpure oxygenxe2x80x9d is intended to include a gas which contains in excess of 85% oxygen by volume.
Preferably the feed gas which is supplied to the slurry contains in excess of 85% oxygen by volume i.e. is substantially pure oxygen.
The method may include the step of maintaining the dissolved oxygen concentration in the slurry within a desired range which may be determined by the operating conditions and the type of microorganisms used for leaching. The applicant has established that a lower limit for the dissolved oxygen concentration to sustain microorganism growth and mineral oxidation, is in the range of from 0.2xc3x9710xe2x88x923 kg/m3 to 4.0xc3x9710xe2x88x923 kg/m3. On the other hand if the dissolved oxygen concentration is too high then microorganism growth is inhibited. The upper threshold concentration also depends on the genus and strain of microorganism used in the leaching process and typically is in the range of from 4xc3x9710xe2x88x923 kg/m3 to 10xc3x9710xe2x88x923 kg/m3.
Thus, preferably, the dissolved oxygen concentration in the slurry is maintained in the range of from 0.2xc3x9710xe2x88x923 kg/m3 to 10xc3x9710xe2x88x923 kg/m3.
The method may include the steps of determining the dissolved oxygen concentration in the slurry and, in response thereto, of controlling at least one of the following: the oxygen content of the feed gas, the rate of supply of the feed gas to the slurry, and the rate of feed of slurry to a reactor.
The dissolved oxygen concentration in the slurry may be determined in any appropriate way, e.g. by one or more of the following: by direct measurement of the dissolved oxygen concentration in the slurry, by measurement of the oxygen content in gas above the slurry, and indirectly by measurement of the oxygen content in off-gas from the slurry, taking into account the rate of oxygen supply, whether in gas enriched or pure form, to the slurry, and other relevant factors.
The method may include the step of controlling the carbon content of the slurry. This may be achieved by one or more of the following: the addition of carbon dioxide gas to the slurry, and the addition of other carbonaceous material to the slurry.
The method may extend to the step of controlling the carbon dioxide content of the feed gas to the slurry in the range of from 0.5% to 5% by volume. A suitable figure is of the order of 1% to 1.5% by volume. The level of the carbon dioxide is chosen to maintain high rates of microorganism growth and sulphide mineral oxidation.
The bioleaching process is preferably carried out at an elevated temperature. As stated hereinbefore the bioleaching rate increases with an increase in operating temperature. Clearly the microorganisms which are used for bioleaching are determined by the operating temperature and vice versa. As the addition of oxygen enriched gas or substantially pure oxygen to the slurry has a cost factor it is desirable to operate at a temperature which increases the leaching rate by an amount which more than compensates for the increase in operating cost. Thus, preferably, the bioleaching is carried out at a temperature in excess of 40xc2x0 C.
The bioleaching may be carried out at a temperature of up to 100xc2x0 C. or more and preferably is carried out at a temperature which lies in a range of from 60xc2x0 C. to 85xc2x0 C.
In one form of the invention the method includes the step of bioleaching the slurry at a temperature of up to 45xc2x0 C. using mesophile microorganisms. These microorganisms may, for example, be selected from the following genus groups:
Acidithiobacillus (formerly Thiobacillus); Leptosprillum; Ferromicrobium; and Acidiphilium.
In order to operate at this temperature the said microorganisms may, for example, be selected from the following species:
Acidithiobacillus caldus (Thiobacillus caldus); Acidithiobacillus thiooxidans (Thiobacillus thiooxidans), Acidithiobacillus ferrooxidans (Thiobacillus ferrooxidans); Acidithiobacillus acidophilus (Thiobacillus acidophilus); Thiobacillus prosperus; Leptospirillium ferrooxidans; Ferromicrobium acidophilus; and Acidiphilium cryptum. 
If the bioleaching step is carried out at a temperature of from 45xc2x0 C. to 60xc2x0 C. then moderate thermophile microorganisms may be used. These may, for example, be selected from the following genus groups:
Acidithiobacillus (formerly Thiobacillus); Acidimicrobium; Sulfobacillus; Ferroplasma (Ferriplasma); and Alicyclobacillus.
Suitable moderate thermophile microorganisms may, for example, be selected from the following species:
Acidithiobacillus caldus (formerly Thiobacillus caldus); Acidimicrobium ferrooxidans; Sulfobacillus acidophilus; Sulfobacillus disulfidooxidans; Sulfobacillus thermosulfidooxidans; Ferroplasma acidarmanus; Thermoplasma acidophilum; and Alicyclobacillus acidocaldrius. 
It is preferred to operate the leaching process at a temperature in the range of from 60xc2x0 C. to 85xc2x0 C. using thermophilic microorganisms. These may, for example, be selected from the following genus groups: Acidothermus; Sulfolobus; Metallosphaera; Acidianus; Ferroplasma (Ferriplasma); Thermoplasma; and Picrophilus.
Suitable thermophilic microorganisms may, for example, be selected from the following species: Sulfolobus metallicus; Sulfolobus acidocaldarius; Sulfolobus thermosulfidooxidans; Acidianus infernus; Metallosphaera sedula; Ferroplasma acidarmanus; Thermoplasma acidophilum; Thermoplasma volcanium; and Picrophilus oshimae. 
The slurry may be leached in a reactor tank or vessel which is open to atmosphere or substantially closed. In the latter case vents for off-gas may be provided from the reactor.
According to a different aspect of the invention there is provided a method of recovering nickel from a slurry containing nickel bearing sulphide minerals which includes the steps of bioleaching the slurry using a suitable microorganism at a temperature in excess of 40xc2x0 C., controlling the dissolved oxygen concentration in the slurry within a predetermined range, and recovering nickel from a bioleach residue.
The said dissolved oxygen concentration may be controlled by controlling the supply of oxygen to the slurry.
The oxygen may be supplied to the slurry in the form of oxygen enriched gas or substantially pure oxygen.
The said operating temperature is preferably above 60xc2x0 C. and may be in the range of 60xc2x0 C. to 85xc2x0 C.
The invention also extends to a method of enhancing the oxygen mass transfer coefficient from a gas phase to a liquid phase in a nickel bearing sulphide mineral slurry which includes the step of supplying a feed gas containing in excess of 21% oxygen by volume to the slurry.
The feed gas preferably contains in excess of 85% oxygen by volume.
The invention further extends to a method of bioleaching an aqueous slurry containing nickel bearing sulphide minerals which includes the steps of bioleaching the slurry at a temperature above 60xc2x0 C. and supplying gas containing in excess of 21% oxygen by volume to the slurry to maintain the dissolved oxygen concentration in the slurry in the range of from 0.2xc3x9710xe2x88x923 kg/m3 to 10xc3x9710xe2x88x923 kg/m3.
The invention is also intended to cover a plant for recovering nickel from a nickel bearing sulphide mineral slurry which includes a reactor vessel, a source which feeds a nickel bearing sulphide mineral slurry to the vessel, an oxygen source, a device which measures the dissolved oxygen concentration in the slurry in the vessel, a control mechanism whereby, in response to the said measure of dissolved oxygen concentration, the supply of oxygen from the oxygen source to the slurry is controlled to achieve a dissolved oxygen concentration in the slurry within a predetermined range, and a recovery system which recovers nickel from a bioleach residue from the reactor vessel.
The plant may include an installation for recovering nickel from lateritic ores by pressure acid leaching to produce a nickel laterite residue slurry which is combined with the said bioleach residue from the reactor, and the combined slurry and residue are fed to the said recovery system.
Various techniques may be used for controlling the supply of oxygen to the slurry and hence for controlling the dissolved oxygen concentration in the slurry at a desired value. Use may for example be made of valves which are operated manually. For more accurate control use may be made of an automatic control system. These techniques are known in the art and are not further described herein.
As has been indicated oxygen and carbon dioxide may be added to the slurry in accordance with predetermined criteria. Although the addition of these materials may be based on expected demand and measurement of other performance parameters, such as iron(II) concentration, it is preferred, however, to make use of suitable measurement probes to sample the actual values of the critical parameters.
For example use may be made of a dissolved oxygen probe to measure the dissolved oxygen concentration in the slurry directly. To achieve this the probe is immersed in the slurry. The dissolved oxygen concentration may be measured indirectly by using a probe in the reactor off-gas or by transmitting a sample of the off-gas, at regular intervals, to an oxygen gas analyser. Again it is pointed out that measuring techniques of this type are known in the art and accordingly any appropriate technique can be used.
A preferred approach to the control aspect is to utilise one or more probes to measure the dissolved oxygen concentration in the slurry, whether directly or indirectly. The probes produce one or more control signals which are used to control the operation of a suitable valve or valves, e.g. solenoid valves, automatically so that the supply of oxygen to an air stream which is being fed to the slurry is varied automatically in accordance with real time measurements of the dissolved oxygen concentration in the slurry.
Although it is preferred to control the addition of oxygen to a gas stream which is fed to the slurry a reverse approach may be adopted in that the oxygen supply rate to the reactor vessel may be maintained substantially constant and the rate of supply of the sulphide mineral slurry to the reactor vessel may be varied to achieve a desired dissolved oxygen concentration.
The invention is not limited to the actual control technique employed and is intended to extend to variations of the aforegoing approaches and to any equivalent process.
Nickel bearing sulphide flotation concentrates frequently contain violarite and pyrrhotite and the method of the invention is of particular benefit, because both pyrrhotite and violarite have a high leaching rate, even at typical mesophile operating temperatures, which is further increased at the higher temperatures used with moderate and extreme thermophiles. Thus the benefits of the invention, including a high specific reactor sulphide oxidation duty and reduced specific power requirement for oxidation, will be obtained during the bioleaching of nickel bearing sulphide concentrates, even at mesophile operating temperatures.
Nickel may be recovered from solution by any appropriate process, for example by solvent extraction applied to the solution or by resin-in-pulp applied to the slurry, followed by electrowinning. The route adopted by Anaconda Nickel for treatment of leach liquors arising from nickel laterites at Murrin Murrin is also relevant i.e. by precipitation of a sulphide (using hydrogen sulphide) and refining of the sulphide precipitate by re-leaching, purification and hydrogen reduction. The route adopted by Preston Resources on a similar solution to that produced at Murrin Murrin at the Cawse project is also applicable, by precipitating a hydroxide (with magnesia), re-leaching, purification using solvent extraction and then electrowinning. Lastly the same process used at Cawse but with the production of a nickel carbonate from the solvent extraction strip liquor, would be most suitable for the production of nickel rondelles by methods established by Queensland Nickel Limited.
If electrowinning is selected as the production method for nickel, the oxygen generated at the anode in the electrowinning process may be used to supplement that used in the bioleach process, reducing the capital and operating costs required for oxygen production.